A permanent magnet generator/motor generally includes a rotor assembly having a plurality of equally spaced magnet poles of alternating polarity around the outer periphery of the rotor or, in more recent times, a solid structure of samarium cobalt or neodymium-iron-boron. The rotor is rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. Alternately, if an electric current is passed through the stator coils, the energized coils will cause the rotor to rotate and thus the generator will perform as a motor.
One of the applications of a permanent magnet generator/motor is referred to as a turbogenerator which includes a power head mounted on the same shaft as the permanent magnet generator/motor, and also includes a combustor and recuperator. The turbogenerator power head would normally include a compressor, a gas turbine and a bearing rotor through which the permanent magnet generator/motor tie rod passes. The compressor is driven by the gas turbine which receives heated exhaust gases from the combustor supplied with preheated air from recuperator.
A permanent magnet turbogenerator/motor can be utilized to provide electrical power for a wide range of utility, commercial and industrial applications. While an individual permanent magnet turbogenerator may only generate 24 to 50 kilowatts, powerplants of up to 500 kilowatts or greater are possible by linking numerous permanent magnet turbogenerator/motors together. Standby power, peak load shaving power and remote location power are just several of the potential utility applications which these lightweight, low noise, low cost, environmentally friendly, and thermally efficient units can be useful for. To meet the stringent utility requirements, particularly when the permanent magnet turbogenerator/motor is to operate as a supplement to utility power, precise control of the permanent magnet turbogenerator/motor is, however, required.
In order to start the turbogenerator/motor, electric current is supplied to the stator coils of the permanent magnet generator/motor to operate the permanent magnet generator/motor as a motor and thus to accelerate the gas turbine of the turbogenerator. During this acceleration, spark and fuel are introduced in the correct sequence to the combustor and self-sustaining gas turbine conditions are reached.
An example of a turbogenerator/motor start-up system is described in U.S. patent application Ser. No. 837,600 filed Apr. 21, 1997 by Edward C. Edelman entitled "Gas Turbine Engine with Fixed Speed Light-Off System and Method", assigned to the same assignee as this application and incorporated herein by reference.
At this point, the inverter is disconnected from the permanent magnet generator/motor, reconfigured to a controlled 50/60 hertz mode, and then either supplies regulated 50/60 hertz three phase voltage to a stand alone load or phase locks to the utility, or to other like controllers, to operate as a supplement to the utility. In this mode of operation, the power for the inverter is derived from the permanent magnet generator/motor via high frequency rectifier bridges. A microprocessor can monitor turbine conditions and control fuel flow to the gas turbine combustor.
An example of such a turbogenerator/motor control system is described in U.S. patent application Ser. No. 924,066, filed Sep. 8, 1997 by Everett R. Geis and Brian W. Peticolas entitled "Turbogenerator/Motor Controller", assigned to the same assignee as this application and incorporated herein by reference.
A typical gas turbine engine of a turbogenerator/motor operates as a constant speed. The gas turbine engine is accelerated to the maximum sustained operating speed and loaded as required. Electronic voltage regulation will maintain a constant voltage. Operating the turbogenerator/motor at a constant speed does not, however, result in maximum operating efficiency for the gas turbine engine.